OBJECTIVE:
The purpose of the present study was to review a group of patients with meniscal
injuries resulting from structural failure unrelated to trauma or degenerative
problems to which was given the name "meniscal injury due to fatigue".MATERIAL AND METHOD: Evaluations were made on 140 patients with meniscal
injuries without any apparent cause, who were therefore considered to have meniscal
injuries due to fatigue. Among these, 85 patients were male and 55 were female.
The medial meniscus was the most affected site (92% of the cases).RESULTS: All these injuries were diagnosed by means of clinical examination
and magnetic resonance imaging. The patients underwent meniscectomy by means
of arthroscopy and the results were divided into two types: good and poor. Poor
results were found in 27% of the cases, among which nine patients progressed
to idiopathic osteonecrosis.CONCLUSION: We conclude that injuries due to fatigue must be assessed
as injuries caused by failure and, therefore, constituting a syndromic pathological
condition that may progress to idiopathic osteonecrosis.

Keywords:
Meniscus. Fatigue. Osteonecrosis.

INTRODUCTION

Knee menisci have
a critical role in absorbing and distributing load. These are anatomical structures
exposed to trauma and aging, just as the structures composing a joint.

Meniscal injury
may occur:

- as a part of
rotational or flexion trauma,

- as an evolution
of joint degeneration process, or

- as a spontaneous
injury resulting from progressive structural failure, without correlation with
trauma or degenerative process.1

Despite of its
various etiologies, symptoms, clinical manifestations and treatment are similar.
When associated to knee instability or to late-stage arthrosis, meniscal injury
is assessed as a result of a more important pathology.

The group of patients
presenting meniscal injury resulting from structural failure with no relation
to trauma, or to degenerative problems, we chose to name it as meniscal injury
due to fatigue because of the absence of correlation with an evident causal
factor and of the similarity of symptoms of fractures due to fatigue.1

the objective of
this study is to assess this group of patients with meniscal injury due to fatigue,
their characteristics and evolution after meniscectomy.

MATERIALS AND
METHODS

Initially, we studied
the evolution of treatment on 435 patients with isolated meniscal injury submitted
to arthroscopic meniscectomy at the Institute of Orthopaedics and Traumatology
at University of São Paulo's Hospital das Clínicas. These patients
presented at baseline with signs of meniscal injury, confirmed by magnetic resonance
imaging (MRI).

All patients with
isolated meniscal injury were submitted to X-ray images at front and lateral
positions of their affected knees; patients with signs of osteoarthrosis at
X-ray such as joint pinching and axis deviation were excluded from this study.

Of the 435 patients,
261 were males (60%) and 174 were females (40%). The prevalent age group (in
decades) was 50-59 years (34.7%). The most frequently affected side was the
right one (53.3%). The most commonly injured meniscus was the medial one (81.8%),
with lateral meniscus being injured in 18.2% of the patients. Both menisci were
injured in 6% of the patients. The distribution by age group is described on
table 1.

The clinical examination
was based on joint interline palpation maneuvers: at first with knee flexed
at 90 degrees and then associated to flexion-extension movements.

Meniscal injury
diagnosis was clinically provided and confirmed by MR (Figure
1) and by arthroscopy during meniscectomy in all patients. (Figure
2)

the kind of meniscal
injury according to its location varied a lot according to the etiology, but
no correlation was clearly established, except for patients with the so-called
injury due to fatigue.

We considered three
groups of patients according to the etiology based on clinical history of meniscal
injury:

I - traumatic injury
- constituted of patients with a clear clinical history of trauma reported in
a given moment.

II - degenerative
injury - constituted of patients with history without an accurate onset of symptoms.
The complaint was insidious and progressive at first.

III - injury due
to fatigue - constituted of patients reporting an acute onset of symptoms, without
trauma or effort justifying injury.

Table
2 shows the distribution of patients according to injury etiology.

One hundred and
forty patients had meniscal injuries with no apparent cause and were regarded
as having meniscal injury due to fatigue, being the object of this study.

The characteristics
of this group of patients were the following :

- Gender: 85 male
patients and 55 female patients

- Affected side:
The right side was affected in 84 patients; the left one was affected in 56.

- Medial meniscus
was the most involved one, in 92% of the cases (128 patients) and the lateral
meniscus was involved in 8% of the cases (12 patients).

In this group there
were no cases of involvement of both menisci.

The kind of medial
meniscal injury was radial at the transition between posterior horn and meniscal
body in 98 patients (76.5%); all other injuries were located at the posterior
horn. Lateral meniscal injuries were classified as many kinds, thus a more frequent
type could not be identified.

The distribution
by age group is described on table 1 and the graphical representation
of the distribution by age groups is shown on Figure 3.

The patients were
submitted to meniscectomy via arthroscopy and were followed up for at least
48 months.

Follow up was provided
by means of visits at intervals of at least half a year, in which the patients
were assessed by one of the authors.

RESULTS

We considered only
two kinds of outcomes: good - meaning a significant symptom improvement and
return to pre-injury activities; poor - no symptoms or complications improvement.

We considered the
evolution of patients in the three groups divided by etiology at the completion
of 48 months.

Table
3 describes the results of the treatment provided in these patients with
isolated meniscal injury by arthroscopic meniscectomy.

Considering the
group in study (injury due to fatigue), we found poor results in 27% of the
cases. Of these, nine evolved to idiopathic osteonecrosis of the knee at femoral
condyle, and all these injuries were at medial femoral condyle.

DISCUSSION

Isolated meniscal
injury more commonly occurs in patients at the fifth decade of life or in older
patients. It is a very frequent event in medical offices, deserving our attention
for some years.2

By assessing the
group of 435 patients submitted to meniscectomy due to isolated meniscal injuries,
140 cases (32% of our sample) called our attention, which have been labeled
as of unclear etiology. The most interesting fact was that nine patients evolved
to femoral condyle osteonecrosis belonging to this group.

We chose to study
this group separately and compared to the overall group in order to check the
possibility of clearly identifying patients with meniscal injury due to fatigue.

In classical identification
items, we found that medial meniscus injuries are more frequent in the fatigue
group than in the global group. The kind of injury (radial) occurring in 76.5%
of the cases was quite characteristic, because we could not establish an injury
pattern in the global group.

Other items like
age, gender, and side are similar to the ones found for the global group.

When studying isolated
traumatic meniscal injuries in young patients, Terzidis et al.3 describe
vertical injuries in 77.5% of the cases. They do not mention radial injuries
in patients having traumatic meniscal injuries. The authors found that, in the
group of patients with traumatic injuries, 30% of the injuries occurred on lateral
meniscus.3

Harper et al.5
describe the challenges of providing a diagnoses of radial meniscal injury by
MR. By comparing the arthroscopic findings, the authors found that 37% of the
patients with radial meniscal injury, as confirmed by arthroscopy, had their
injuries undiagnosed by MR.

We believe that
radial medial meniscal injury, despite of its difficult diagnosis, is almost
a pattern among patients with meniscal injury due to fatigue.

By assessing the
results of treatment in the global group, we found that traumatic meniscal injuries
evolve well after treatment by arthroscopic partial meniscectomy. We found 92%
of good outcomes in our cases.

Meniscectomy in
degenerative cases provided satisfactory results in only 57% of the cases, but
we have already noticed these results.1

By studying the
evolution of patients with degenerative knee meniscal injury, Herrlin et al.6
compared conservative treatment with muscular balance to the treatment by arthroscopic
partial meniscectomy and concluded that the results were similar.

Our patients with
injury due to fatigue had good results in only 73% of the cases, and nine patients
evolved to idiopathic osteonecrosis of the medial femoral condyle; perhaps,
this is the worst complication of arthroscopic partial meniscectomy.7
(Figure 4)

Muscolo et al.8
describe five cases of osteonecrosis after medial meniscectomy and conclude
that this complication may occur in older patients.

Pape et al.9
correlated the occurrence of osteonecrosis with patients' age and osteoporosis
and not with meniscectomy in six patients.

We couldn't characterize
age as an important factor in the genesis of injury due to fatigue as well as
in the occurrence of idiopathic osteonecrosis.

Zanetti et al.10
found a correlation between the so-called idiopathic knee osteonecrosis with
osteoporosis in 32 patients, eight of them males.

We didn't study
the occurrence of osteoporosis in patients with idiopathic osteonecrosis in
our sample.

By studying the
histology of 14 femoral condyles with osteonecrosis, Yamamoto et al.11
found that bone necrosis does not occur, but fracture by stress, with signs
of callus formation and repair tissue. In fact, there is no necrosis but a fracture
by stress resulting from a structural failure.

Amatuzzi et al.12
describe knee osteonecrosis and suggest trauma as a potential etiology. The
authors also correlate the occurrence of osteonecrosis with meniscal injury.

Nakamura et al.13
describe a case of subchondral fracture of the femoral condyle after meniscectomy.
The authors found in this case an identical image to the so-called idiopathic
osteonecrosis at MR.

The occurrence
of idiopathic osteonecrosis in patients with signs of structural failure without
correlation with meniscectomy is relatively frequent.

NaryClez et al.14
describe the association of femoral condyle osteonecrosis with fracture by stress
in four patients. In the group of patients studied here, none had been submitted
to meniscectomy.

Idiopathic osteonecrosis
symptoms are identical to those of the meniscal injury due to fatigue: Sudden
onset of pain without traumatic cause12; and the MR image is equal
to that found in patients who had idiopathic osteonecrosis after meniscectomy.

Patients with osteonecrosis
due to vasculitis or to other causes have completely different symptoms. MR
images of the so-called secondary osteonecrosis are totally distinct, even for
its location, which is more frequently seen at lateral condyle.

We believe that
the so-called idiopathic knee osteonecrosis is a result of structural failure
process that led the meniscus to develop a radial injury. We understand that
the so-called idiopathic osteonecrosis would be a fracture of the femoral condyle
due to fatigue and should be named as fracture by failure or fracture due
to fatigue. The name osteonecrosis seems to be inappropriate, since there
is no osteonecrosis in a large portion of the cases, being confused with osteonecrosis
resulting from other non-traumatic pathologies.7

CONCLUSION

The group of patients
with the so-called meniscal injury due to fatigue should be assessed as patients
presenting the first symptoms of failure of osteocartiginous structure of knee
joint. They should be treated as having a syndromic pathology that may evolve
to idiopathic osteonecrosis.